Quarterly report pursuant to Section 13 or 15(d)

Fair Value - Schedule of Fair Value of Financial Assets and Liabilities Inputs (Details)

v3.8.0.1
Fair Value - Schedule of Fair Value of Financial Assets and Liabilities Inputs (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Assets    
Total assets measured at fair value $ 312,665 $ 354,553
Money market funds    
Assets    
Total assets measured at fair value 105,865 353,303
Commercial paper    
Assets    
Total assets measured at fair value 194,383  
Corporate debt securities    
Assets    
Total assets measured at fair value 12,417 1,250
Quoted Prices in Active Markets for Identical Instruments (Level 1)    
Assets    
Total assets measured at fair value 105,865 353,303
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Money market funds    
Assets    
Total assets measured at fair value 105,865 353,303
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Commercial paper    
Assets    
Total assets measured at fair value 0  
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Corporate debt securities    
Assets    
Total assets measured at fair value 0 0
Significant Other Observable Inputs (Level 2)    
Assets    
Total assets measured at fair value 206,800 1,250
Significant Other Observable Inputs (Level 2) | Money market funds    
Assets    
Total assets measured at fair value 0 0
Significant Other Observable Inputs (Level 2) | Commercial paper    
Assets    
Total assets measured at fair value 194,383  
Significant Other Observable Inputs (Level 2) | Corporate debt securities    
Assets    
Total assets measured at fair value $ 12,417 $ 1,250